The present invention relates to a method and apparatus for the improved recovery of C.sub.3 and heavier components from hydrocarbon gases.
In conventional processes for extracting propane and heavier components from hydrocarbon gases, the C.sub.3 bearing gases are treated by a combination of expansion (or compression followed by expansion) heat exchange and refrigeration to obtain a partially condensed stream which is collected in a feed separator having a pressure typically in the order of 50 to 300 psia and a temperature in the order of -50.degree. to -200.degree. F. These conditions of course can vary substantially, depending on the pressure and temperature conditions necessary to achieve partial condensation for a particular gas, and the pressure and temperature at which the feed is available to the process. The liquid resulting from partial condensation is supplied to a fractionation column called a "deethanizer" as a mid-column feed while the vapor from the feed separator is used to generate reflux by partially condensing the overhead vapors from the deethanizer through appropriate heat exchange means. In a typical system the deethanizer column will operate at a pressure substantially equal to that of the deethanizer feed separator (possibly allowing for a small pressure drop as the partially condensed liquid passes from the separator to the deethanizer) and the deethanizer overhead vapors leave at a temperature in the order of -20.degree. to -70.degree. F. The heat exchange of these overhead vapors against the residue vapors from the low pressure separator provides partial condensate which is used as a reflux to the deethanizer column.
Pre-cooling of the gas before it is expanded to the deethanizer pressure will commonly result in formation of a high-pressure condensate. To avoid damage to the expander, the high pressure condensate, if it forms, is usually separated, separately expanded through a Joule-Thomson valve and used as a further feed to the mid-portion of the deethanizer column. Refrigeration in such a process is sometimes entirely generated by work expansion of the vapors remaining after partial condensation of the high pressure gas to the column operating pressure. Other processes may include external refrigeration of the high pressure gases to provide some of the required cooling.
When processing natural gas, feed is typically available at line pressure, of 900-1000 psia. In such case expansion to a pressure in the order of 250-300 psia is common. In an alternate process, facilities may be designed to extract propane or propylene from refinery gases. Refinery gases commonly are available a pressure of 150 psia-250 psia. In this case, at the convenience of the process designer, the deethanizer may be designed to operate at a pressure below the pressure of the refinery gas which is available, i.e., perhaps 50-100 psia, so that work expansion can be used to supply refrigeration to the process. This will result in lower deethanizer temperatures and will increase potential heat leakage and other engineering problems associated with cryogenic temperatures. It is also possible in this case to compress the refinery gas to a higher pressure so that it may be thereafter expanded in a work-expansion machine to afford refrigeration to the overall process.
A typical flow plan of a process for separating C.sub.3 and heavier hydrocarbons from a gas stream is illustrated in U.S. Pat. No. 4,251,249 to Jerry G. Gulsby.